(1) Field of the Invention
The present invention relates to the field of piloting rotary wing aircraft such as rotorcraft during stages of flight at low power.
This may apply in particular in the event of a failure of a power plant that was initially driving rotation of a main rotor of a rotorcraft for the purpose of providing it with lift. This also happens during a stage of flight in auto-rotation in which the kinetic energy of the rotorcraft descending serves to drive its main rotor in rotation, thereby giving rise to a lift force.
Thus, the invention relates more particularly to a method of controlling a rotorcraft main rotor, which method is adapted to enable such a rotorcraft to perform a stage of flight in auto-rotation. The invention also provides a control system for a rotorcraft main rotor, and a rotorcraft fitted with such a control system.
(2) Description of Related Art
Usually, the failure of a power plant and a stage of flight in auto-rotation are two situations that occur together. Specifically, it is generally as a result of a failure of at least one engine that a pilot of the rotorcraft changes over as an emergency to a stage of flight in auto-rotation in order to enable the rotorcraft to land as soon as possible.
Nevertheless, it is necessary for all pilots to be trained in performing such difficult emergency maneuvers. A stage of flight in auto-rotation can thus also be performed voluntarily by the pilot with engines that are operational but delivering little or no power to the main rotor.
In order to trigger such a stage of flight in auto-rotation, the pilot of the rotorcraft generally actuates a control member such as a collective pitch stick serving to modify the pitch of the blades of the main rotor of the rotorcraft collectively. By way of example, the stage of flight in auto-rotation may then be triggered voluntarily by a pilot placing the control member in such a manner as to desynchronize rotation of the main rotor and rotation of the engines. The pilot continues to move the control member manually until reaching a level of collective pitch that makes it possible to perform a stage of flight in auto-rotation, which level is referred to below as the “auto-rotation collective pitch”. Furthermore, this “auto-rotation collective pitch” is a function of the size and of the number of blades making up the rotor.
Thus, such a required “auto-rotation collective pitch” may be determined by calculation at the time the main rotor is designed. The “auto-rotation collective pitch” then corresponds to the collective pitch angle of the blades that enables the main rotor to store kinetic energy in rotation so as to guarantee that the flight path for the rotorcraft can be flattened out for landing on the ground without impact. Such stored kinetic energy is then transformed into potential energy to enable the rotorcraft to be landed without danger by reducing its vertical speed on landing, possibly even to zero.
In addition, during such an auto-rotation maneuver, the current pitch level is displayed on an on-board instrument referred to as a “first limit indicator” (FLI). The FLI also serves to indicate both a “desynchronization” pitch between the rotor and the power plant, and also a maximum/minimum pitch that is acceptable for the power plant driving the blades of the rotor in rotation.
Nevertheless, under such circumstances, the visual display on the FLI does not make it possible to avoid unwanted desynchronization as a result of the pilot making an inappropriate maneuver on the control member, such as a collective pitch stick. Nor does the display on the FLI help in balancing the speed of auto-rotation, and in particular help avoid unwanted resynchronization between the rotor and the power plant. Specifically, the change over from the synchronized state to the desynchronized state needs to be stable in order to reduce the workload on the pilot. Finally, it is not possible to guarantee that the power delivered by the power plant will not exceed limits.
A pilot does indeed regularly monitor the rotorcraft's instruments, but the pilot also needs to observe the outside of the rotorcraft, very particularly during difficult maneuvers, such as stages of flight in auto-rotation and/or landing approaches after a steep flight path. By looking outside, the pilot might miss seeing limit violations on the scale of the FLI.
Furthermore, it is also known to provide a rotorcraft with a system for identifying an engine failure and then automatically triggering a stage of flight in auto-rotation. Such a control method and system are thus described in Documents US 2013/0221153 and US 2007/164167.
Nevertheless, such a control system is limited to an automatic safety system and does not appear to apply to the situation in which a pilot of the rotorcraft seeks voluntarily to trigger a stage of flight of the rotorcraft in auto-rotation.
Furthermore, all of those devices seek to provide protection so as to avoid power plant power limits being violated at high levels of collective pitch, e.g. corresponding to a collective pitch angle lying in the range 7° to 10°. Nor do they guarantee safety for a stage of flight in auto-rotation at low power levels, e.g. corresponding to a collective pitch angle lying in the range 0° to 3°.
Document FR 2 766 158 describes a method and a system for controlling a rotorcraft that provide a pilot with assistance in avoiding reaching a maximum engine speed. Such a method thus generates a motor-driven stop forming a hard point opposing freely continued pivoting movement of the collective pitch lever in a downward direction when the setpoint maximum rotor speed is reached or exceeded. Such a motor-driven stop is also servo-controlled in position and it can be exceeded.
Nevertheless, in Document FR 2 766 158, once the motor-driven stop has been exceeded, it does not facilitate piloting the rotorcraft in auto-rotation by servo-controlling the position of the collective pitch lever on a predetermined position referred to as the “auto-rotation position”. The collective pitch lever therefore does not act automatically to generate a control setpoint for servo-controlling the current collective pitch of the blades of the main rotor on an auto-rotation collective pitch.
Furthermore, Document FR 2 766 158 does not disclose that the motor-driven stop coincides with a “desynchronization position” in which the collective pitch lever generates a control setpoint for servo-controlling a current collective pitch of the blades of the main rotor on a “desynchronization collective pitch” enabling the rotary motion of the main rotor and of a power plant of the rotorcraft to be desynchronized.
Document US 2004/010354 also describes a method enabling a pilot to be assisted by means of a tactile sense signal transmitted by motor-driven means to a mini-stick for controlling the collective pitch.
Nevertheless, as in Document FR 2 766 158, such a Document US 2004/010354 does not describe servo-controlling the position of the mini-stick on the auto-rotation position, nor does it describe the exceedable stop coinciding with a desynchronization position.
Consequently, such a method is not optimum for assisting the piloting of a rotorcraft during a stage of flight in auto-rotation.